The present invention relates to a catalyst powder, a catalyst electrode, and an electrochemical device.
Solid polymer electrolyte type fuel cells can have a higher energy density as compared with other types of fuel cells, and are considered to be easy to reduce in size; therefore, solid polymer electrolyte type fuel cells are very highly expected to be applied to portable uses. Accordingly, the solid polymer electrolyte type fuel cells constitute a field in which developments have recently been vigorously carried out in the world and to which much attention has been being paid.
The cell structure of a solid polymer electrolyte type fuel cell is a laminate structure of cathode current collector/cathode (oxygen electrode) side catalyst layer/solid polymer type electrolyte/anode (fuel electrode) side catalyst layer/anode current collector, and a variety of researches have been carried out as to the materials of the components thereof, the manufacturing methods thereof, the methods for forming the cell structures thereof, and the like (see, for example, Japanese Patent Laid-open No. Hei 5-36418 (page 2, right lower column, line 42 to page 3, column 3, line 9)).
In addition, the solid polymer electrolyte type fuel cell is supplied with oxygen on the cathode side and with a fuel such as hydrogen and an alcohol on the anode side, to thereby generate electric power.
The catalyst layer on the cathode or anode side is comprised of a solid polymer electrolyte, and a catalyst powder including particles of a catalyst such as platinum supported on a carbon powder, and it is considered that an optimum ratio of the catalyst particles is in the range of 20 to 55 wt % (see, for example, Japanese Patent Laid-open No. Hei 8-117598 (page 3, column 3, lines 12 to 22). The optimum ratio has been considered to be in the above-mentioned range, taking into account the effective utilization efficiency of the catalyst particles such as platinum particles in view of the amount used, the particle diameter, and the specific surface are of the catalyst particles.
However, in order to enlarge the areal density of the catalyst particles in the catalyst layer for the purpose of increasing the output current, there has been no other way than enlarging the thickness of the catalyst layer. Where a catalyst powder with a catalyst particle supporting ratio of 20 to 55 wt % as above-mentioned is used, an increase in the thickness of the catalyst layer leads to an increase in the amount of a binder needed, because the specific surface area of the catalyst particles is enlarged, and results in an increase of the electronic resistance in the electrode. Besides, forming a thicker catalyst layer involves such problems as cracking of the layer, resulting in that the stability of the catalyst layer is lowered.
On the other hand, where a catalyst powder with a high catalyst particle supporting ratio is used, the specific surface area of the catalyst particles is small, so that a sufficient reaction area cannot be obtained, and good output characteristics cannot be developed.